1. Field of the Invention
This invention relates to an electro-optical display device.
2. Description of Prior Art
A number of display devices operated by an electric field have been developed, including those based on the electrochromic phenomenon. One typical example is set forth in British Pat. No. 1,186,541 and U.S. Pat. No. 3,521,941 which comprises a pair of electrodes at least one of which is optically transparent between which are disposed an electrochromic layer containing the oxide of a transition metal or copper and an insulating intermediate layer capable of transmitting the electric charge carrier therethrough. Such a device operates by altering its electromagnetic absorption characteristics in response to an electric signal thus developing or erasing color. The devices set forth in the above-cited patents specify a single component in each of the electrochromic and insulating layers.
Japanese Patent Application (OPI) Nos. 87343 to 87347/1975 describe a variety of display devices also based on electrochromism wherein the following, various layer arrangements are disclosed. A layer comprising two or more transition metal compounds superposed with a layer of an electrolyte, a single layer structure containing both of the transition metal compound and the electrolyte dispersed therein; a double layer structure comprising a layer containing a metal oxide together with a metal and an electrolyte layer; a single layer structure containing two or more of transition metal compounds and an electrolyte, a liquid electrolyte layer, a layer comprising a porous material containing electrolytes, etc.
Furthermore, Japanese Patent Application (OPI) NO. 73749/1977 discloses a double layer structure comprising an electrolytically reducible chromogenic thin layer and an electrolytically oxidizable thin layer disposed between a pair of electrodes. Each of these electrochromic display devices has a common shortcoming. With repeated color development and erasing color reversibility deteriorates, and seriously limits the life of the display device. Accordingly, such display devices have been regarded as inadequate and unreliable. On the other hand, the so-called memory effect, i.e., the ability to retain the once developed color for an extended period, as is well known, is rather poor. Another disadvantage of such devices is that a relatively high operating level of current density is required to maintain the colored state unchanged.